The report concerns the development and validation of a simulation model for estimating ground level concentrations of photochemical pollutants. It describes limited model development and improvement; extensive evaluation of the photochemical kinetics mechanism, involving fourteen experimental studies and four hydrocarbon systems; and extensive evaluation of the urban airshed model for the Los Angeles Basin for six days for five pollutants: carbon monoxide, nitric oxide, nitrogen dioxide, hydrocarbon, and ozone. All technical aspects of the work are discussed including: the urban airshed model, its theoretical foundation and its formulation, as well as with the modifications and improvements made, and the validation of the model--procedures, results, and discussion and evaluation of results.